Physiological mechanisms, often regulated by central monoaminergic systems, direct sexual development and behavior in all vertebrates. Catecholamines regulate the hypothalamo-pituitary-gonad axis, while serotonin dictates dominance hierarchies. Sex-changing fish undergo a total reorganization of the reproductive system and sex-specific behaviors in a single individual. Monoamine neurotransmitters will be investigated to elucidate physiological mechanisms underlying sex reversal in fishes and their role in controlling sexual development and sex-specific behavior in general. While previous studies have investigated the association of gonadal steroids or gonadotropin-releasing hormone with sex reversal, I am interested in the upstream regulation of these factors via monoamines. I will use a sex-changing coral reef fish, Thalassoma duperrey, to address the following questions: 1) Are monoamines involved in sex reversal? 2) Which areas of the brain are associated with monoamines and sex reversal? 3) Do monoamines have a causative role in sex reversal and associated behaviors? 4) What are the basic patterns of monoamine synthesis and metabolism in a sex changing species? By investigating these questions in a system that has the capacity to change from male to female, and has two male phenotypes, we can gain insight into basic questions of neural correlates of sex determination and sex-specific behaviors across various taxa. Because social structure serves to initiate sex reversal in these fish, the social structure can be manipulated to induce sex reversal. Using the techniques of HPLC, immunohistochemistry and behavioral observations, I will document changes in monoamines and behavior associated with sex reversal in this fish.